Analog electronic filter circuits are an integral part of a wide variety of electronic and semiconductor products. Filter circuits generally perform a predetermined function of preventing and permitting certain signal frequencies to be communicated. Analog filter circuits are characterized by being implemented with components such as resistors and capacitors. Resistors and capacitors can each be chosen to have one of a wide variety of component values. However, all integrated circuit analog component values, including resistors and capacitors, are subject to variation of the component value as a function primarily of temperature and processing (i.e. manufacturing) variations. Therefore, tuning circuits may be used with analog filter circuits in order to fine tune or adjust the filter to compensate for variation in the filter's analog components.
One well known category of analog filters is the switched capacitor filter, so called because only switches and capacitors are used with an amplifier. An advantage of the switched capacitor filter is that resistors, which can vary in value, are not used. Also, the time constant is proportional to capacitor ratios as opposed to absolute analog capacitance values. Although capacitors also vary in value, capacitors can be ratioed and matched in a manner to minimize the effects of variation. In switched capacitor filters, amplifier frequency bandwidth must be much greater than an input signal sampling frequency, and the filter's input signal sampling frequency must be at least twice greater than the filter's pole frequency. However, switched capacitor filters are subject to frequency aliasing errors, require an additional filter stage known as an anti-aliasing filter and a smoothing filter, and have performance degradations at higher frequencies.
Another known category of analog filters is the continuous time filter. One known implementation of the continuous time filter uses both capacitors and resistors and an amplifier. In a continuous time filter, the frequency response is directly proportional to variation of the values of resistors and capacitors. Since these values vary greatly, the frequency response also varies. Therefore, a tuning circuit is often required for a continuous time filter.
Yet another known category of analog filters is the operational transconductance amplifier-capacitor (OTA-C) filter. An operational transconductance amplifier filter uses the transconductance (gm) of an operational amplifier as the resistance required to implement the filter. However, the transconductance of an amplifier is subject to nonlinear variation. Operational transconductance amplifiers also have relatively poor dynamic range and must be tunable.
A wide variety of known filter tuning circuits exists. A common method of filter tuning is to laser trim the resistors or capacitors, or both. A primary disadvantage with circuits having components which must be trimmed is the cost associated with the additional manufacturing and production. Another common tuning circuit which is used in connection with analog filters is the use of off-chip external discrete resistors and capacitors which are less susceptible to variation and which may be readily adjusted to tune the filter. A disadvantage with this method is the requirement for components external to a filter integrated circuit. External components add cost, increase integrated circuit pin count and provide opportunities for noise and error to be introduced into the filter operation. A further common tuning circuit is integrated with the filter, but uses switched capacitors to simulate resistors. Capacitors can be ratioed with one another to minimize the effects of varying capacitances. However, noise at the switching frequency is commonly introduced in this type of tuner. Yet another type of known filter tuning circuit is the phase lock loop technique which uses a phase detector, a loop filter, and a voltage controlled oscillator (VCO) to tune the filter. Phase lock loops require a large amount of circuitry in relation to the filter and typically introduce noise at a reference frequency provided for the phase lock loop. The reference frequency must also be equal to the filter's pole frequency. Therefore, additional clock frequency circuitry may be required to provide a specific reference frequency.